1. Field of the Invention
The present invention relates to a touch signal probe for measuring a surface position of a workpiece by contact, especially a vibrating touch signal probe for detecting vibration change caused when a contact portion of a stylus touches the workpiece. More specifically, it relates to a contact portion detecting mechanism of a vibrating touch signal probe for detecting a contact location of the contact portion against the workpiece.
2. Description of the Related Art
A height gauge (linear measuring machine), a coordinate measuring machine and a profile measuring machine are known as a measuring machine for measuring the configuration and dimensions of a workpiece. Various probes are used in the measuring machine for detecting the positional relationship of a measuring machine body and the workpiece. The probes can be grouped into a non-contact probe and a contact probe, or alternatively, a continuous measuring probe and a touch trigger probe.
An ultrasonic touch trigger probe disclosed in Japanese Patent Laid-Open Publication No. Hei 6-221806 is known as a contact touch trigger probe for the above coordinates measuring machine. As shown in FIG. 10, the touch signal probe 100 includes a stylus holder 101, a stylus 102 and a piezoelectric element 103. The stylus holder 101 is mounted to a stylus support moving in a three-dimensional space at a predetermined velocity vector in accordance with an external command (not shown). The stylus moves along with the stylus support and detects the contact against the workpiece, so that an edge position of the workpiece is detected by reading the coordinate in contact.
The stylus holder 101 is shaped into a tubular cylinder, and the stylus 102 is supported at the inner lower end thereof by a pair of engage pins 104 provided on a support point at an approximate axial center of the stylus 102. A contact portion 102A having a spherical surface to be abutted to the workpiece in measurement is attached to a lower end of the stylus 102 and a counter balance 102B having the same weight as the contact portion 102A is mounted to an upper end of the stylus 102, so that the support point of the stylus holder 101 is consistent with a centroid of the stylus 102.
A pair of grooves 105 as a attachment portion of the piezoelectric element 103 are cut on an outer circumference of the stylus 102 and two piezoelectric elements 103 having the same configuration are attached to the grooves 105 by an adhesive or the like with both ends being firmly adhered.
The piezoelectric elements 103 are disposed approximately symmetrically along an axial direction of the stylus 102 with the support point of the stylus 102 at the center thereof, the piezoelectric elements being divided into a vibrator 103A for resonantly vibrating the stylus 102 and a detector 103B for detecting a change in vibration of the stylus 102.
The touch signal probe 100 vibrates the stylus 102 along an axial direction thereof by the vibrator 103A. When the ball-shaped contact portion 102A touches the workpiece, vibration of the stylus 102 is restricted by a contact force to change vibration status. Accordingly, edge position etc. can be detected by detecting the change in the vibration status by the detector 103B.
However, since the change in the vibration status of the stylus 102 differs according to a difference in contact location (the part of the surface of the ball-shaped contact portion 102A actually touching the workpiece) of the contact portion 102A against the workpiece and deflection by the contact force of the contact portion 102A, the touch signal probe 100 having the above-described arrangement is not suitable for a probe for conducting profiling measurement and continuous measurement where various parts of the contact portion 102A touches the workpiece.
Specifically, as shown in FIG. 11(A), when the contact portion 102A touches the workpiece W in a direction orthogonal with the axial direction (vibrating direction) of the stylus 102, the vibration status along the axial direction of the stylus 102 does not largely change even when the deflection against the workpiece W by a contact force F1 of the contact portion 102A increases.
On the contrary, as shown in FIG. 11(B), when the contact portion 102A touches the workpiece W along the axial direction of the stylus 102, the vibration status of the stylus 102 largely changes even when the deflection toward the workpiece W by contact force F2 of the contact portion 102A changes only slightly. The status is shown in FIG. 12, where a detection signal V showing vibration status change of the stylus 102 changes gently relative to a change in deflection L as shown in graph G10. On the other hand, the detection signal V largely changes even against a small change of the deflection L.
Accordingly, when the touch signal probe 100 is moved together with the stylus support in a three-dimensional direction to conduct profiling measurement for measuring the configuration of the workpiece W, since the change in vibration status of the stylus 102 largely differs according to the contact location of the contact portion 102A, non-ambiguous determination of the edge of the workpiece W is difficult according to magnitude of the vibration status change of the stylus 102. Therefore, such ultrasonic touch signal probe 100 is not suitable as a probe for profiling measurement and continuous measurement of the workpiece W. Further, when the size of the probe is reduced, the rigidity of the stylus is reduced. Accordingly, axial resonance of the stylus cannot be maintained in the continuous measurement, thus resulting in decrease in sensitivity, i.e. deterioration in accuracy.